In PCCR eye tracking, the gaze vector of an eye is determined on the basis of on an image of the eye when illuminated in such manner that reflections (glints) appear on the cornea. Glint positions and the pupil center position are extracted from the image using generic computer-vision methods. Methods for computing the gaze vector based on these positions are known in the art, e.g., through the teachings of E. D. Guestrin and M. Eizenmann in IEEE Transactions on Biomedical Engineering, Vol. 53, No. 6, pp. 1124-1133 (June 2006), included herein by reference.
An important application of PCCR eye-tracking technology is the task of finding the gaze point of a person watching a visual display. Since visual displays are artefacts constructed generally with the aim of providing optimal viewing conditions in terms of luminance, viewing distance and angle, image contrast etc., it might be expected that the measurement accuracy is very high in this situation, particularly when the eye tracking is performed indoors with a controlled ambient illumination.
In many practical cases, however, a considerable unreliability is introduced by the difficulty to provide illuminators that are not unsuitably distant from the expected gaze point. Indeed, the reflection created by an oblique illuminator may fall on the sclera, outside the cornea, and since the sclera has spherical shape with respect to the eye's center of rotation, this reflection is not useful for determining the orientation of the eye.
There have been attempts to place illuminators on the display screen surface. Measurements according to this approach may not always give authentic results, because each illuminator acts a visible stimulus and perturbs the natural behaviour of the person.
Other attempts include arranging illuminators on the border of the visual display, that is, outside the screen surface on which the display is adapted to create visual images. This means that the border cannot be made narrow, contrary to normal aesthetic wishes. This difficulty is accentuated if a two-dimensional array of illuminators is to be provided on each border segment, which is desirable for an accurate two-dimensional position measurement of the cornea. Combining reflections from illuminators arranged on opposing borders of the display is usually not feasible, for it is only in a narrow range of viewing angles, near the center, that reflections from both borders fall on the cornea.
Third, interlacing the visual display image with a geometrically distinct reference pattern for creating corneal reflections has been tried. Unless a display dedicated for producing both visible images and an invisible reference pattern is used, the reference pattern is generated by visible light. The interlacing may be performed intermittently during short time intervals, which are synchronised with the intervals for measuring the corneal reflection of the reference pattern. A common difficulty in implementing this approach is that the time intervals, however short, may need to occur rather frequently to achieve sufficient signal power of the reference pattern. Then, because of the time-integrating functioning of the retina, a perceptible superimposed image of the reference pattern may be produced and distract the subject.
Hence, for gaze tracking in connection with visual displays, there is a need for improved illuminators not suffering from the problems outlined above.
Two further shortcomings are inherent in many known PCCR implementations. First, the processing involved in finding the pupil center in an eye image may be problematic. For subjects having a dark iris color, particularly in the absence of a retinal reflection, the faint pupil-to-iris contrast can make the pupil boundary difficult to discern with a limited computational effort. Second, as noted in the already cited article by Guestrin and Eizenmann, the approximation of the cornea as a spherical surface is an important source of errors. Indeed, it has long been known in the art of visual optics that the cornea rather has an ellipsoidal shape, and it would be desirable to achieve improved illuminators for eye-tracking that represent a progress also with respect to these shortcomings.